The present invention relates generally to electrical water heaters. More particularly, the invention relates to methods and apparatus for pulsing electrical power to electrical resistance heating elements in a manner to improve the heating efficiency of the water heater.
A storage-type water heater typically comprises a permanently enclosed water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating material in at least a portion of the annular space for providing thermal insulation to the water tank. The water tank has various appurtenances such as inlet, outlet and drain fittings. Additionally, the water heater is provided with a water heating and temperature control system. The water heating and temperature control system includes an electrical-resistance-heating element. The heating element extends through a fitting in the wall of the water tank such that the heating element is inside the tank. The heating element is connected to an electrical power source outside the water tank.
Conventional water heating and temperature control systems typically further include a mechanical thermostat. The mechanical thermostat closes a switch to allow electrical power through the electrical resistance heating element when water in the tank is sensed to be below a selected set-point temperature, and opens the switch to stop electrical power from passing through the electrical resistance heating element when the water in the tank is at or above the set point temperature. Electrical power through the electrical resistance heating element is either fully on, passing full electrical current, or completely off. Due to variations in manufacture and hysteresis of the mechanical thermostat, the temperature of the water will xe2x80x9covershootxe2x80x9d the desired set-point temperature. In other words, the water heating and temperature control system allows the electrical resistance heating element to continue heating water in the water tank even when the water temperature is above the set point temperature. It would be beneficial to prevent or limit the amount of overshoot of the conventional water heater.
Accordingly, the invention provides a water heater having a controller for modulating electric power to an electrical-resistance-heating element in controllable pulses or bursts. Providing electric power to the heating element in pulses or bursts allows an equal amount of water to be heated to a selected temperature at substantially the same rate as with a mechanical temperature controller of the prior art, yet uses substantially less electric power to heat the water. Therefore, modulating the electric power improves the efficiency of the water heater.
A preferable way for modulating electric power in short bursts to the resistance heating element is by using a proportional band temperature controller that takes into account the unique signature of the water heater. That is, when calculating the amount of modulation between a burst of electric power being supplied to a heating element and a period during which no electric power is supplied to the element, the water heater may vary the amount of modulation based on a number of variables or water heater characteristics. The variables may include, but are not limited to: heating element wattage, element watt density, location of the heating element(s), number of elements mounted within a tank, operating voltage of the water heater, inlet water temperature, water capacity of the water tank, ambient room temperature of the physical environment in which the heater is installed, and usage patterns of the facility in which the heater is being used. By combining all of these aspects with proportional band technology, significantly greater energy savings are achieved over conventional electric water heaters.
The invention further provides a water heater including a tank for holding water, a water inlet line having an inlet opening that introduces cold water to the tank, a water outlet line having an outlet opening that withdraws heated water from the tank, and a first heating element extending into the tank. The water heater further includes a control circuit operable to control the supply of electric power to a heating element in bursts. Each burst is followed by a period during which electric power is not supplied to the heating element.
In one embodiment, the tank has a tank characteristic, the heating element has an element characteristic, and the control circuit includes a temperature sensor operable to sense a temperature of the water within the tank. The control circuit further includes a controller in communication with the heating element and the temperature sensor. The controller is operable to receive the sensed temperature from the temperature sensor, to calculate a heating strategy for the water heater based in part on the element characteristic and/or the tank characteristic, and to generate a signal activating the heating element in response to the heating strategy. In another embodiment, the control circuit is further operable to change the proportion of on to off time in response to the sensed water temperature and at least one of an element characteristic, a tank characteristic, an external water tank temperature, a water consistency, and a history of water use.
The invention even further provides a method of controlling a temperature of water in a water heater. The method includes the acts of determining an element characteristic of the heating element, sensing a temperature of the water in the tank, calculating an amount of power to be provided to the heating element based at least in part on the element characteristic and water temperature, and transmitting the amount of power from the power source to the heating element. The calculating act may also be based at least in part on a tank characteristic, an environment (i.e., ambient) temperature, or a water characteristic (i.e., temperature, hardness, etc.).
The invention further provides a software program for generating a signal resulting in an amount of power to be transmitted to a heating element. The software program generates the signal by obtaining a water heater code from a memory unit. The water heater code is based at least in part on a heating element characteristic or on a tank characteristic. The software program further includes receiving the temperature of the liquid from the temperature sensor, calculating the amount of power to transmit to the heating element based at least in part on the water heater code and the sensed temperature, and generating the signal resulting in the amount of power being provided to the heating element.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.